1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the same. In particular, the present invention is suited to an image forming apparatus such as a laser beam printer, a digital copying machine, or a multifunction printer, which employs an electrophotographic process.
2. Description of the Related Art
In a laser beam printer, a digital copying machine, a multifunction printer, or other image forming apparatus (image output portion) employing an electrophotographic process, scanning is performed on a photosensitive member (photosensitive drum) with a light flux (laser light flux) that blinks in accordance with print data.
Thus, exposure distribution is formed, and image formation is realized by an electrostatic copying mechanism in accordance with the exposure distribution.
In recent years, a resin material is frequently used for imaging optical elements to be used for a scanning optical apparatus so as to achieve high precision and a simple structure of the scanning optical apparatus.
When the resin material is used for the imaging optical element, molding of the resin can be performed so that lenses and mirrors having an aspheric surface can be manufactured easily.
Further, if a casing for holding a shape of the lenses and mirrors is also made of the resin molding, lighter weight and a simpler structure can be realized, and a flexible lens arrangement can be achieved, compared with a conventional casing made by metal cutting or casting.
Various types of such scanning optical apparatus have been proposed conventionally (see Japanese Patent Application Laid-Open No. 2001-194611).
In order to perform high precision recording of image information in such scanning optical apparatus, it is necessary that a curvature of field be corrected appropriately over an entire area of a surface to be scanned (image surface), and that an angle of field (scanning angle) θ and an image height Y have distortion characteristic (fθ characteristic) accompanied with isokinetic property.
Further, it is necessary that a spot diameter on the surface to be scanned be uniform with respect to each image height.
Recently, along with higher resolution of a laser beam printer, imaging performance of higher image quality is required. Therefore, it is required to mount a component with a micron order precision of the position when the apparatus main body is assembled.
However, a manufacturing error of a component, a mounting positional error in the assembling process, and other various errors may occur when the main body is assembled, which may cause fluctuation in performance of the products. As a typical problem caused by the fluctuation, there is a problem of a shift of an imaging position due to a tilt of imaging field generated in a deflection surface of an optical deflector.
Usually, the tilt of imaging field in the deflection surface can be canceled by disposing an imaging optical element so that an object point on a deflection surface and an image point on the photosensitive drum have a conjugate relationship in the scanning optical apparatus.
Therefore, a cylindrical lens, which is disposed for a purpose of condensing light on the deflection surface in an incident optical unit for forming the object point on the deflection surface, has an important role in forming the object point on the deflection surface.
Concerning the cylindrical lens, a shift due to an error in an optical axis direction does not cause a change of the irradiation position, but other error in the other direction (in yz plane) shows up as a shift of the irradiation position on the deflection surface. Consequently, the image point is shifted in a main scanning direction and in a sub scanning direction on the image surface.
In addition, some correction is performed finally for the fluctuation in optical performance due to the above-mentioned error.
For instance, correction of a scanning line bend is corrected by bending a scanning line in the same direction by the same degree for each color so that a color drift does not occur in the developing step.
For this reason, it is necessary to decrease an influence of an error as much as possible in the light ray (light flux) to be a reference for correction.
However, such attention has not been given to the optical element of the incident optical system in the conventional scanning optical apparatus.
Recently, the attention as described above has become an indispensable for realizing a micro spot of the light flux or multi-beam thereof, which requires high assembly precision.
Therefore, it is important to design so that errors that may occur in components or in the assembly process can be reduced as much as possible.